The present invention relates to a method for removing acid gases from a fluid stream, e.g. for removing carbon dioxide from flue gases.
In numerous processes in the chemical industry, fluid streams occur which contain acid gases such as, e.g., CO2, H2S, SO2, CS2, HCN, COS or mercaptans. These fluid streams can be, for example, gas streams such as natural gas, refinery gas, synthesis gas, flue gases, or reaction gases formed in the composting of waste materials comprising organic substances. The removal of the acid gases from these fluid streams is desirable for various reasons.
The removal of carbon dioxide from flue gases serves, in particular, to reduce the emission of carbon dioxide, which is considered to be the main cause of what is termed the greenhouse effect.
Synthesis gas comprises substantially carbon monoxide and hydrogen. Synthesis gas is generally produced by partial oxidation or steam reforming of hydrocarbons. The crude synthesis gas comprises acid gases such as carbon dioxide, hydrogen sulfide, or carbonyl sulfide, which must be removed.
The content of acid gases in natural gas is reduced by suitable processing measures directly at the natural gas well, since these acid gases, in the water frequently entrained by the natural gas, form acids which are corrosive.
On the industrial scale, aqueous solutions of organic bases, e.g. amines, such as, in particular, alkanolamines, are frequently used as absorption medium for removing acid gases, such as carbon dioxide, from fluid streams. On the dissolution of acid gases, ionic products are formed from the base and the acid gas components. The absorption medium can be regenerated by warming, expanding to a lower pressure, or stripping, wherein the ionic products react back to form acid gases and/or the acid gases are stripped off by means of steam. After the regeneration process, the absorption medium can be reused.
However, the amines have a vapor pressure which is not negligible. Therefore, the fluid stream freed from acid gases comprises traces of amines. Contamination of the treated fluid stream is undesirable for various reasons. For instance, it is disadvantageous when traces of amines escape into the environment together with the treated flue gas.
Synthesis gas is the starting material of further catalytic reactions. Amine traces can act here as catalyst poison.
The content of amines in natural gas or liquefied petroleum gas (LPG) produced therefrom by liquefaction can likewise be subject to restrictions.
It has been proposed in the prior art to scrub the treated fluid stream with an aqueous phase in order to transfer entrained amine at least in part into the aqueous phase.
EP 0 798 029 A2 discloses a method in which a gas is treated with a basic amine compound for absorbing carbon dioxide, and the treated gas is then contacted at 20 to 60° C. with an aqueous phase in order to transfer entrained basic amine at least in part into the aqueous phase. The aqueous phase should preferably be condensate which is condensed out of the carbon dioxide which is released in the regeneration tower.
US 2008/0159937 comprises a method for removing carbon dioxide from a gas stream, in which the gas stream that is depleted in carbon dioxide is scrubbed with water in a packed section of the absorption tower. The water can be condensate from the top of the regeneration column, or fresh water for making up lost amounts.
The use of condensate which is condensed out of the carbon dioxide which is released in the regeneration tower as scrubbing water has the advantage that the water balance of the absorption medium circuit is not impaired. On the other hand, the condensate is only available in a limited amount. Fresh water can be used as scrubbing water only in a restricted amount in order not to dilute the absorption medium and accumulate water in the method. The amount of fresh water to be added results from the difference between the water content of the incoming fluid stream and the exiting streams. For the configuration shown in FIG. 1, for example, the incoming stream 1 and the two exiting streams 21 and 25 enter into the water balance. The fresh water required to make up lost amounts is termed make-up water (stream 11 in FIG. 1). The exiting streams are saturated with water vapor. The water content of the incoming fluid stream depends on various conditions. For a water-saturated incoming fluid stream, the makeup stream available decreases with decreasing pressure difference between the absorption and the regeneration. Consequently, in particular in the case of flue gas scrubbers in which the absorption pressure is close to atmospheric pressure, the amount of make-up water available is restricted.
In order, in the case of a restricted amount of scrubbing water, nevertheless to achieve an adequate scrubbing action, it has been proposed to conduct the scrubbing water through the scrubbing zone not in a single pass, but to circulate the scrubbing water by pumping, or to recycle it, i.e. to collect it below the scrubbing zone and reapply it above the scrubbing zone. Optionally, the scrubbing water in this case can be conducted via an additional cooler. By means of the cooling, water condenses out of the treated fluid stream. In order to avoid accumulation in the scrubbing water circuit of absorption medium components that have been scrubbed out, a subquantity of the scrubbing water is discharged and replaced by make-up water. The water discharged from the scrubbing water circuit is customarily passed into the absorption medium circuit.
In Satish Reddy et al., Fluor's Econamine FG PlusSM Technology, presented at the Second National Conference on Carbon Sequestration, National Energy Technology Department of Energy, Alexandria Va., USA, May 5-8, 2003, a typical embodiment of a gas scrubbing process having a scrubbing zone with scrubbing water circulated by pumping is described.
By means of the recycling and optional cooling of the scrubbing water, the scrubbing action can be increased. Customary volume ratios of recycled scrubbing water and make-up water are between 10 and 500. However, owing to the recycling, a backmixing of the scrubbing water occurs. At very high volume ratios of recycled scrubbing water and make-up water, in the scrubbing zone only the action of at most one theoretical separation plate can be achieved, independently of the length of the contact section in the scrubbing zone.
WO 2010/102877 describes a method in which the gas stream that is depleted in carbon dioxide is scrubbed with an acidic aqueous solution in order to decrease the amount of amines and basic degradation products present therein. Between the carbon dioxide absorption zone and the acidic scrubbing, a scrubbing with water can be provided.